CVE-2025-6121: D-Link DIR-632 Buffer Overflow Vulnerability

CVE-2025-6121 Overview

A critical stack-based buffer overflow vulnerability has been identified in the D-Link DIR-632 router running firmware version FW103B08. The vulnerability resides in the get_pure_content function within the HTTP POST Request Handler component, where improper handling of the Content-Length argument allows attackers to corrupt memory on the stack. This flaw enables remote unauthenticated attackers to potentially execute arbitrary code or cause denial of service on affected devices. Critically, this vulnerability affects a product that is no longer supported by D-Link, meaning no official patch will be released.

Critical Impact

Remote attackers can exploit this stack-based buffer overflow to potentially achieve full device compromise, intercept network traffic, or pivot to other devices on the network. The affected product is end-of-life with no vendor support available.

Affected Products

• D-Link DIR-632 Firmware version 103B08
• D-Link DIR-632 Hardware (all revisions running affected firmware)

Discovery Timeline

• 2025-06-16 - CVE-2025-6121 published to NVD
• 2025-06-17 - Last updated in NVD database

Technical Details for CVE-2025-6121

Vulnerability Analysis

This vulnerability is a classic stack-based buffer overflow (CWE-119: Improper Restriction of Operations within the Bounds of a Memory Buffer) affecting the HTTP POST request processing functionality of the D-Link DIR-632 router. The flaw occurs in the get_pure_content function, which handles incoming HTTP requests. When processing POST requests, the function fails to properly validate the Content-Length header value against the actual allocated buffer size on the stack.

The attack can be launched remotely over the network without requiring authentication, making it particularly dangerous for devices exposed to the internet or untrusted network segments. Successful exploitation could allow an attacker to overwrite critical stack data, including return addresses, potentially leading to arbitrary code execution with the privileges of the web server process.

Root Cause

The root cause lies in insufficient bounds checking when the get_pure_content function reads HTTP POST body content. The function allocates a fixed-size buffer on the stack but uses the attacker-controlled Content-Length header to determine how many bytes to read. By supplying a Content-Length value larger than the allocated buffer, an attacker can write beyond the buffer boundaries, corrupting adjacent stack memory including saved return addresses and other critical data structures.

Attack Vector

The attack is conducted remotely over the network by sending a specially crafted HTTP POST request to the router's web management interface. The attacker manipulates the Content-Length header to specify a value larger than the stack buffer can accommodate. When the vulnerable function processes this request, it reads the specified number of bytes into the undersized buffer, causing a stack overflow.

The attacker does not require any authentication or user interaction to exploit this vulnerability. Any network-accessible instance of the D-Link DIR-632 running firmware version FW103B08 is potentially vulnerable.

A proof-of-concept demonstrating this vulnerability has been publicly disclosed. Technical details are available in the GitHub PoC Repository.

Detection Methods for CVE-2025-6121

Indicators of Compromise

• Unexpected router reboots or crashes, indicating potential exploitation attempts
• Unusual outbound network connections from the router to unknown IP addresses
• Anomalous HTTP POST requests with excessively large Content-Length headers in router logs
• Modified router configuration or firmware without administrator action
• Presence of unauthorized user accounts or changes to administrative credentials

Detection Strategies

• Monitor network traffic for HTTP POST requests targeting D-Link DIR-632 devices with abnormally large Content-Length header values
• Implement network-based intrusion detection rules to identify buffer overflow exploitation patterns
• Deploy network segmentation to isolate vulnerable end-of-life devices from critical network assets
• Review router logs for suspicious administrative access or configuration changes

Monitoring Recommendations

• Set up alerts for HTTP requests to the router management interface from untrusted network segments
• Implement continuous monitoring of device availability to detect crash-based exploitation attempts
• Monitor for firmware integrity changes using periodic checksum verification
• Track unusual traffic patterns originating from or directed at affected D-Link devices

How to Mitigate CVE-2025-6121

Immediate Actions Required

• Immediately restrict access to the router's web management interface to trusted IP addresses only
• Disable remote management features if not absolutely required
• Place the vulnerable router behind a firewall that can filter malicious HTTP requests
• Consider replacing the end-of-life D-Link DIR-632 with a currently supported router model
• Isolate the device on a separate network segment from critical infrastructure

Patch Information

No official patch is available for this vulnerability. D-Link has discontinued support for the DIR-632 router, and the company has indicated that no security updates will be released for this end-of-life product. Organizations using this device are strongly advised to replace it with a currently supported model.

For additional information, visit the D-Link Official Website or review the VulDB Vulnerability Analysis.

Workarounds

• Disable the web management interface entirely and manage the device through console access only
• Implement strict access control lists (ACLs) to limit which IP addresses can reach the management interface
• Deploy a web application firewall (WAF) or intrusion prevention system (IPS) in front of the device to filter malicious requests
• Use VPN access for remote management instead of exposing the interface directly
• Schedule replacement of the end-of-life device with a supported alternative as soon as possible

# Example: Firewall rule to restrict access to router management interface
# Replace 192.168.1.1 with your router's IP and 192.168.1.100 with trusted admin IP

# iptables example (on upstream firewall)
iptables -A FORWARD -d 192.168.1.1 -p tcp --dport 80 -s 192.168.1.100 -j ACCEPT
iptables -A FORWARD -d 192.168.1.1 -p tcp --dport 80 -j DROP
iptables -A FORWARD -d 192.168.1.1 -p tcp --dport 443 -s 192.168.1.100 -j ACCEPT
iptables -A FORWARD -d 192.168.1.1 -p tcp --dport 443 -j DROP